System for measuring turbulence remotely

ABSTRACT

A system and method for detecting turbulence includes several mobile platforms, a mobile platform velocity sensor, and several electromagnetic energy transmitters and receivers. The receivers receive the energy transmitted by the transmitter(s) after it has traveled along a path subject to the turbulence. The receivers detect alterations of the energy caused by the turbulence and filter the alterations for effects of the mobile platform velocity (on which either a transmitter or receiver is located). Additionally, the system may create a three-dimensional model of the. In another preferred embodiment, the present invention provides a method of detecting turbulence using a mobile platform. The method includes receiving electromagnetic energy that has traveled along a path subject to the turbulence and determining the alteration to the energy caused by the turbulence. The alterations are filtered of the effects of the velocity of the mobile platform on which the receivers are preferably located.

FIELD OF THE INVENTION

This invention relates generally to meteorological sensors and, moreparticularly, sensors that sense clear air turbulence remotely.

BACKGROUND OF THE INVENTION

Clear air turbulence significantly affects the comfort of passengers oncommercial aircraft and has even caused some would be passengers toforego flying due to their fears associated with the turbulence. Becauseclear air turbulence can occur with little or no warning, the passengerstend to stay in their seats with their seat belts fastened. At times,though, every passenger must get up for comfort and physiologicalreasons. Therefore, if the aircraft must proceed through the turbulence,it would be useful if the aircrew could alert the passengers to thedisturbance before the aircraft encounters it.

Preferably, the aircraft would avoid the turbulence altogether but eventhat preventative measure requires that the turbulence be detected orpredicted before it occurs. While much turbulence (e.g. the turbulenceassociated with thunderstorms) can be predicted or detected, clear airturbulence can not be detected or predicted by currently availabletechnology. The reason that clear air turbulence cannot be detected isthat it consists of masses of air having slightly differenttemperatures, pressures, and densities moving at various speeds anddirections in the atmosphere. The minute differences in these air massesdo not reflect radar differently enough to make the radar return fromone mass of air distinguishable from the radar return from another massof air.

While meteorological maps provide flight crews some indication of whereturbulence might be expected, these maps are not perfect. First, theytend to become stale within hours and are based on underlyingmeteorological models that are far from perfect also. Additionally,turbulence occurs across a wide variety of geometric scales. Someturbulent areas can extend for many kilometers, or even hundreds ofkilometers (e.g. the turbulent region surrounding the jet stream). Otherareas of turbulence occur on the scale of kilometers or fractions ofkilometers such as the turbulence associated with the downstream side ofa mountain that is subjected to brief wind gusts of significantvelocity. Due to their scale, these smaller volumes of turbulence willnot appear on the meteorological maps.

In the absence of any better approach, the aviation industry has createda system in which the pilots of each aircraft radio in reports of theclear air turbulence that they encounter on their routes, or “airways.”Subsequent aircraft flying the same airway can maneuver in response tothese reports but risk encountering turbulence along their detour.Obviously, the first aircrew to fly along a given airway after theairway has been vacant for some time will have no reports on which tobase evasive action. Likewise, those aircraft on unplanned detours suchas when an airport is too busy to accept arrivals, or is otherwise shutdown (by for example severe weather), will have no way to foresee theturbulence along the route.

SUMMARY OF THE INVENTION

Apparatus and methods for remotely sensing turbulence, particularly aclear air turbulence meter, provide a system that measures atmosphericturbulence along a line of sight between a receiver and a satellite. Thesystem uses alterations to a signal (that include, but are not limitedto changes in intensity, phase, and frequency) that is transmitted fromthe satellite to the receiver to make the turbulence measurement. In oneembodiment, the receiver is a GPS receiver that estimates thecontribution of ionospheric scintillation to the signal alterations byusing the GPS L1 and L2 bands. Preferably, these ionosphere effects areremoved from the alteration to isolate the effects of troposphericturbulence on the signal.

Other preferred embodiments are adapted for use on land and marinevehicles and include velocity sensors such as inertial measurement unitsthat enable the receiver to adjust the turbulence measurement to accountfor the motion of the vehicle. In the alternative, the system caninclude an input for receiving velocity information from the vehicle.These vehicle-adapted systems can determine velocity-induced phaseshifts and Doppler effects from the velocity of the vehicle and removethese effects from the measured variations of the signal. Also, thesystem can include an input to receive the heading of the vehicle toenable the system to determine the direction to each GPS satellitecurrently in view. The direction can be determined relative to theaircraft heading or relative to the ground (or Earth). Further, thesystem can adjust the measured turbulence estimate for crosswind effects(i.e. apparent turbulence introduced into the measurement because of themotion of the receiver relative to the turbulent volumes of air).Moreover, signals from more than one satellite constellation (e.g. GPS,GLONASS, and Galileo) can be used by the receiver to make themeasurements. Using more than one constellation improves theavailability of transmitted signals, gives better coverage of theatmosphere, and improves the accuracy of the turbulence measurements.The turbulence measurements can be conveyed to end users such as theaircrew, air traffic controllers or computers, or other aircraft. Theforms in which the turbulence measurements can be conveyed includeaudible alarms, overlays of turbulence intensity on aircrew stationdisplays, or overlays of turbulence intensity on a map. Thus, airlinesoperating in accordance with the principles of the present inventionwill provide smoother flights with fewer occurrences of passengers beingadvised to return to their seats because of the possibility ofturbulence. Moreover, the number of times when the advisories are basedon inaccurate predictions (e.g. “false alarms”) will be reduced.Likewise, detours of aircraft around turbulence will be avoided therebyreducing fuel consumption.

In a second preferred embodiment, the present invention provides areceiver of electromagnetic energy (that travels along a path that issubject to turbulence). The receiver includes an input, an output, and acircuit in communication with the input and the output. The inputreceives a first signal that is representative of the electromagneticenergy as it is received. The circuit accepts the first signal and asecond signal that is representative of a velocity of a mobile platform.Also, the circuit adjusts the first signal using the second signal todetermine an alteration of the electromagnetic energy caused by theturbulence thereby eliminating alterations caused by the velocity of themobile platform. In a preferred embodiment, the circuit determines thealteration caused by only the tropospheric turbulence. The outputgenerates a third signal that is representative of the turbulence.

The receiver preferably includes a GPS (Global Positioning System), orsimilar circuit, and accepts a fourth signal that is representative of aheading of the mobile platform. From the fourth signal, the receiverdetermines a direction to the source of the electromagnetic energy.Moreover, the circuit may accept yet another signal that isrepresentative of the electromagnetic energy from a second receivinglocation. In these embodiments, the circuit determines from that signala second alteration of the energy caused by the turbulence. In anotherpreferred embodiment, the circuit correlates the two measurements of thealteration caused by the turbulence. More particularly, the receivercorrelates the two measurements with respect to the time it took for anantenna at the second location to move to the first location.

In a third preferred embodiment, the present invention provides a mobileplatform that includes an antenna, a velocity sensor, and anelectromagnetic energy receiver. The antenna receives theelectromagnetic energy (that has traveled along a path subject toturbulence) while the sensor senses the velocity of the mobile platform.Using the sensed velocity, the receiver filters the as-receivedelectromagnetic energy to determine an alteration to the energy that wascaused by the turbulence. The mobile platform may also provide to thereceiver a signal representing a heading of the platform so that thereceiver can determine a direction to the source of the energy. Also,the mobile platform (e.g. an aircraft, a land vehicle, or a marinevehicle) can include a second antenna to receive the electromagneticenergy thereby allowing the circuit to make a second measurement of theturbulence. Additionally, the circuit may correlate the two measurementswith respect to the amount of time it took for the second antenna tomove to the location where the first antenna received the energy.Preferably, the antennas are located on a sidewall of the mobileplatform.

In another preferred embodiment, the present invention provides a systemfor detecting turbulence. In the current embodiment, the system includesat least one mobile platform, a sensor that determines the velocity ofthe at least one mobile platform, at least one electromagnetic energytransmitter, and at least one receiver. The transmitter transmits theelectromagnetic energy across a path that is subject to turbulence andthe receiver receives the energy (even if the transmitter is near thehorizon as seen by the receiver). At least one of the transmitters orreceivers is on the mobile platform. Again, the receiver determines analteration to the energy that is caused by the turbulence. When thereceiver determines the alteration the receiver may also associate atime, a location, and a direction with the determined alteration.Preferably, the system includes a processor that creates a threedimensional model (e.g. a computer aided tomographic model) of theturbulence from the measurements made by the receivers. In turn, anetwork may be used to distribute the model to subscribers in apublish-subscribe architecture. In another preferred embodiment themodel includes a statistical confidence interval. Moreover, the modelmay be supplemented with data from other sources such as air datasensors, inertial sensors on mobile platforms, meteorological sensors,and meteorological predictions. Preferably, the system is configured tosense the turbulence over a pre-selected geographic region such as anairport approach or departure path.

A method of measuring turbulence is provided by yet another preferredembodiment. The method of the current embodiment includes receivingelectromagnetic energy that has traveled along a path subject to theturbulence. The method also includes determining the alteration to theenergy caused by the turbulence by filtering the electromagnetic energy(as it was received) with a signal that represents the velocity ofeither the transmitter or the receiver. An alteration caused by theionosphere may also be filtered from the alteration to theelectromagnetic energy. Preferably, the method includes determining adirection between the receiver and the transmitter. A determination mayalso be made of the alteration caused by the turbulence as measured at asecond location. Further, a three-dimensional model of the turbulencemay be created and distributed to subscribers to the model.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate exemplary embodiments of the presentinvention and together with the description, serve to explain theprinciples of the invention. In the drawings:

FIG. 1 illustrates a global system for the detection of clear airturbulence in accordance with the principles of the present invention;

FIG. 2 schematically illustrates a radio receiver of a preferredembodiment of the present invention;

FIG. 3 illustrates a system architecture for the system of FIG. 1; and

FIG. 4 illustrates a method in accordance with the principles of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Many modern aircraft use radio positioning signals broadcast fromsatellites (e.g. GPS or GLONASS) for navigation. Atmospheric turbulencecan cause the GPS receivers to occasionally lose lock with the signalsby corrupting, or altering, the signal to an extent sufficient to renderthe receiver temporarily inoperative. The problem becomes morepronounced when the transmitting satellite, as seen by the receiver,nears the horizon. Not only does the signal have to traverse asignificantly longer path through the atmosphere, but the signal path islikely to penetrate deeply into the troposphere where turbulence can bemuch more pronounced than in the higher portions of the atmosphere.Also, as the signal path nears the ground, multipathing can occur whichfurther degrades the signal quality. Because turbulence has previouslybeen seen as a problem to be avoided, the receiver antennas aretypically configured to reject signals with low elevation anglesrelative to the horizon.

According to the principles of the present invention, though, thealtered signals carry an indication of the amount of turbulence throughwhich the signals have passed. While any one signal only conveysinformation regarding the turbulence along its path, the large number ofGPS receivers and satellites currently in use provide a plethora ofturbulence measurements along the numerous paths between these devices.By a process similar to tomography (e.g. computer aided tomography orCAT), these turbulence measurements can be used to create athree-dimensional model of the turbulence in the atmosphere.

Before turning to a more detailed description of the invention, it isuseful to discuss the structure of the atmosphere as it relates toturbulence. The lowest portion of the atmosphere is the troposphere andis the volume of air where most commercial and military aviation occurs.The troposphere begins at the surface of the Earth and, during the day,is composed of a surface boundary layer, a mixing layer, an entrainmentlayer, and the lowest reaches of the “free” atmosphere. The surfaceboundary layer, mixing layer, and entrainment layer typically extend upto about 1 to 3 kilometers. These layers are sometimes collectivelyreferred to as the planetary boundary layer because effects offrictional drag with the surface of the Earth can be observed in theselayers. In contrast to these lower levels of the atmosphere, the effectsof the ground are negligible, or nonexistent, in the “free” atmosphere.

Because it is the layer of the atmosphere in direct contact with theEarth, the surface boundary layer (which is about 10% of the planetaryboundary layer) is dominated by mechanical shear between the air and theground and outright obstructions to the movement of the air (e.g.mountains or buildings). These interactions give rise to local eddies onmany scales from millimeters to many hundreds of kilometers. Solarheating and radiative cooling of the air and the ground cause areas ofconvection to develop thereby creating up and down drafts. Thus, winds(i.e. the turbulence) in the surface boundary layer have components inall three dimensions and are not a function of height. Further, strongvertical gradients exist in the properties (e.g. temperature, pressure,and humidity) of the air in this layer.

Being above the surface boundary layer, the mixing layer is influencedby the ground to a lesser extent than the surface boundary layer. Thewinds in the mixing layer are characterized by large scale eddies thatare generally on the scale of many kilometers, or larger. Additionally,plumes of heated air rising from the surface boundary layer and massesof cooler air sinking from the entrainment layer (i.e. tubules) alsoexist in a generally random distribution throughout the mixing layer.Thus, much of the small-scale chaotic flow of the surface boundarydissipates with altitude.

The entrainment layer lies just above the mixing layer. In theentrainment layer, the rising plumes of heated air reach thermodynamicequilibrium with their surroundings and stop rising. Cumulus cloudstherefore form at the tops of these thermal plumes which can reach thetop of the troposphere in extreme cases (e.g. severe thunderstorms).Adjacent to the warm rising plumes of air, masses of cooler denser airare displaced and sink into the mixing layer.

At night heating from solar radiation stops as radiative cooling of theground and air begins to predominate. Thus, the energy that drives thedaytime turbulence fades and allows friction with the surface tostabilize a layer of air near the ground. Another layer of air above the“stable layer” contains residual turbulence left behind by the daytimeatmosphere. The “residual” layer generally corresponds to the mixing andentrainment layers.

Thus, in general, turbulence occurs when the cells of air in the mixinglayer, called turbules, rise and fall through the atmosphere atdifferent rates due to density differences between the turbules and thesurrounding air. Sometimes the turbulence is visible, or detectable withradar, due to precipitation entrained in (or precipitating from) theturbulent air. Often, though, no detectable indication of the turbulenceoccurs so that when an aircraft encounters the turbulence, it appears tocome from the “clear air.”

The density differences between the turbules and surrounding air arelargely a function of temperature, pressure, and humidity although otherproperties of the air in the turbule also vary from that of thesurrounding air. Because of the differing properties, the index ofrefraction of the air in the turbules differs from the index ofrefraction of the nearby air in the mixing layer. Ao has shown that theindex of refraction “n” is related to the properties of air as follows:(n−1)×10⁶ =a ₁ P/T+a ₂ P _(w) /T ²where T is the air temperature, P is the air pressure, P_(w) is thewater vapor pressure (i.e. a measure of humidity), a₁ is 77.6 K mbar⁻¹and a₂ is 3.73×10⁵ K² mbar⁻¹. [Ao, C. O. et al., Lower-TroposphereRefractivity Bias in GPS Occultation Retrievals, Journal of GeophysicalResearch, 108 (D18), Pages 1-12.] As a result, the turbules refractelectromagnetic waves as the waves pass through the turbules. The amountof refraction occurring along a wave's (or signal's) path thereforechanges as turbules move into or out of the signal path. The changingamount of refraction causes several measurable alterations to thesignal. More particularly, these alterations include changes in thephase, the intensity, and the frequency of the wave induced by changesto the path that the signal travels.

Moreover, because the signal path is continuously changing, the signalwill appear to be arriving from different paths. Because the paths havedifferent lengths, it is possible for one instantaneous portion of thewave to partially overtake another instantaneous portion of the wavesignal. Thus, the portions of the wave may interfere eitherconstructively or destructively. The result is higher or lower signalintensity, respectively, at the receiver. Thus, rapid variations inintensity are therefore an indicator of turbulence along the signalpath.

The changing signal paths also give rise to frequency shifts of thesignal. These frequency shifts occur because the effect of the changingpath lengths is the same as if the satellite were actually retreating atthe velocity with which the path length changes. This phenomenon issimilar to the Doppler effect caused by a transmitter and receivermoving relative to each other. Thus, rapid changes in frequency alsoindicate turbulence along the signal path. Previously available GPSreceivers typically measure frequency and use the detected Dopplereffect to compute the receiver's heading and speed. However, theseprevious GPS receivers, by design, smooth out short-term fluctuations togive an accurate average receiver velocity. Thus, the previouslyavailable GPS receivers treat the fluctuations as a problem whereas thereceivers of the current embodiment include frequency detectors thatpick up the signal prior to the averaging function and provide anotherindication of turbulence.

Turning now to the phase shifts caused by the turbules, these shiftsalso occur because at one instant the signal arrives from one path andat the next instant it arrives from a slightly different path. Becausethe different paths will almost always have different lengths, thesignal arriving at one instant will have traveled a different distancethan the signal arriving at another instant. The difference in pathlength causes the signal to undergo a phase shift at one time relativeto the other time. Thus, variations in phase are yet another indicatorof turbulence along the signal path.

Tropospheric turbulence is not the only source of alteration to (i.e.scintillation of) signals transmitted to, or from, space. The Earth'sionosphere also alters the signals in a manner that is stronglydependant on frequency. Thus, the receivers of the present invention usesignals having different frequencies to measure the ionospheric effectson the signals. As a result, the receivers can remove the ionosphericalterations from the signals thereby leaving only the alterations thatare due to tropospheric effects (i.e. troposphere turbulence).

In a preferred embodiment the invention combines the use of high-qualityGPS receivers onboard aircraft to measure signal quality with acomputerized navigation system to compute the relative positions of theaircraft and satellites. The receivers use the GPS signal quality toestimate turbulence between the aircraft and the satellites. If strongturbulence is detected in an aircraft's path, a warning may be issued tothe aircrew. Otherwise, the turbulence measurements can be collected andused to build a three dimensional model of the atmosphere that showswhere turbulence is occurring and the degree to which it is occurring.

Referring to the accompanying drawings in which like reference numbersindicate like elements, FIG. 1 illustrates a global turbulence measuringsystem 10 constructed in accordance with the principles of the presentinvention.

The exemplary system 10 shown in FIG. 1 includes a constellation ofsatellites 12, 14, 16, 18, and 20, a plurality of mobile platforms 22,24, and 26, and a ground station 28 distributed in such a manner as todetect the volumes of turbulence 30 that might occur in the atmosphere.While the turbulence 30 is shown as a cumulonimbus cloud (i.e. athunderstorm) the principles of the present invention apply equally wellto turbulence that bears no visible indication of its presence and toturbulence that cannot be detected by radar. Also, FIG. 1 shows thetroposphere 32 (extending up to an altitude of about 11 miles) and theionosphere 34 (extending up to an altitude of about 400 miles).

The satellites 12, 14, 16, 18, and 20 may be any satellite thattransmits signals in the form of electromagnetic energy (e.g. radiofrequency energy) generally toward the Earth or any other celestial bodyhaving an atmosphere. Preferably, the satellites are components of aconstellation of satellites such as a system for providing globalpositioning services (e.g. the Global Positioning System, GLONASS, orGalileo systems), a system for providing telecommunications (e.g. theIridium, Globalstar, Intermediate Circular Orbit, Orbcomm, or Teledesicsystems), or even a collection of unrelated satellites. Likewise, theparticular mobile platforms 22, 24, and 26 used are not critical. Butexemplary mobile platforms include aircraft 22 and 24 and ships 26 aswell as other air, space, marine, and land vehicles. Preferably, eachsatellite carries a transmitter to broadcast signals for receipt byreceivers at the terrestrial portions 22, 24, 26, and 28 of the system10 although the location of the receivers and transmitters can bereversed or interchanged without departing from the scope of the presentinvention.

The transmission of the signals between the transmitters and receiversis illustrated by a variety of signal paths in FIG. 1. For instance,satellite 12 is shown transmitting two signals received by the aircraft22 and one signal received by the ship 26 via, respectively paths 36, 38and path 40. Satellite 14 is also shown transmitting to the aircraft 22via path 42. Likewise, satellite 16 is transmitting to the aircraft 24via path 44 and satellite 18 is transmitting to the ship 26 via path 46.As is apparent from FIG. 1, each of the receiving portions of the system10 can receive one, or more, signals.

The majority of these paths 36, 38, 40, 42, 44, and 46 will pass throughboth the ionosphere 34 and the troposphere 32 while being altered byconditions in each of these portions of the atmosphere. Thesealterations will typically include instant-to-instant phase shifts,frequency shifts, and intensity changes in the signal as it is receivedat the terrestrial portions 22, 24, 26, and 28 of the system 10. Manyportions of the system 10 move. Thus, the paths 36, 38, 40, 42, 44, and46 will sweep through the atmosphere forming curvilinearthree-dimensional surfaces along which the signals travel during thetime that any pair of transmitters and receivers are visible to one andother. As the mobile components of the system 10 move, the paths willencounter varying degrees of turbulence 30. For example, paths 36, 38,40, and 24 are shown traversing relatively stable portions of theatmosphere while paths 42 and 46 are both shown penetrating the volumeof turbulence 30 albeit at different locations and angles. Thus, theturbulence 30 will alter the signals traveling on the paths 42 and 46 toa greater extent than the atmosphere will alter the signals that travelon the other paths 36, 38, 40, and 44.

With reference now to FIG. 2, a receiver 110 constructed in accordancewith a preferred embodiment of the present embodiment is illustrated inblock diagram form. For perspective, a simplified system 100 is alsoshown and includes a satellite or transmitter 106 broadcasting a signal108 to the exemplary receiver 110. The receiver 110 includes a number ofinputs, outputs, and components as follows: a transmitted signal input112, a signal rejector 114, a signal bypass 116, a signalconditioner/demodulator 118, a phase detector 120, a frequency detector122, an amplitude or intensity detector 124, and a signal processor 126.The receiver 110 also includes an ionospheric turbulence detector 132, arelated inverter 134, and a signal direction finder 136. To interfacewith systems onboard a mobile platform, the receiver 110 also includes amobile platform systems input 127, a phase shift estimator 128, afrequency shift estimator 130, and a pair of related inverters 129 and131. The components of the receiver 110 (and their equivalents) areinterconnected with each other as shown or can be implemented insoftware. Further, the receiver 110 communicates with one, or more,antennas 138 via the input 112 to receive the signals from the satellite106. Also, the receiver 110 communicates with the INS (InertialNavigation System) and FCS (Flight Control System) 140 of the mobileplatform via the input 127. As will be described, the receiver 110generates a turbulence vector at an output 142.

In operation, the transmitter 106 transmits an electromagnetic signal108 that travels along a path that is subject to turbulence. Theturbulence alters the signal 108 thereby causing phase shifts, frequencyshifts, or changes to the intensity (i.e. fading and enhancement) of thesignal as it is received at the antenna 138. The antenna 138 guides thesignal to the signal input 112. If the transmitter 106 is too close tothe horizon, an antenna properly designed for positioning applicationswill typically reject the signal 108 due to the possibility that noisemay corrupt the incoming signal. This feature is shown schematically atthe rejector 114 even though no component that is separate from theantenna 138 is usually required. The present invention seeks theselow-elevation, noisy signals 108, in particular, because they bearuseful indications of the turbulence 30 (see FIG. 1) along the signal's108 path through the atmosphere. Thus, the bypass 116 schematicallyshows the antenna 138 communicating all signals 108 to the signalconditioner 118 even though the signals 108 may be close to the horizon.Again, the bypass function 116 for the noisy signals is typically acharacteristic of the antenna 138 rather than a component separate fromthe antenna 138.

The signal conditioner 118 of FIG. 2 could be divided into two portions:one portion for conditioning the relatively noise-free signals andgenerating position data and another portion conditioning all signalsand supporting the generation of turbulence data. At appropriate nodeswithin the signal conditioner 118, signals are picked up andcommunicated to the detectors 120, 122, and 124. By examining the signal108, the detectors 120, 122, and 124 detect, respectively, phase shifts,frequency shifts, and fading or enhancement of the signal 108. Themagnitude of these alterations and the rates at which they are detectedare fed to the signal processor 126 (or an equivalent analog circuit)that converts the data to an indication of the amount of turbulencealong the path that the signal 108 took in reaching the antenna 138.Generally, the turbulence will be proportional to a combination of thealterations to the signal 108 caused by the turbulence.

Adjustments may also be made to the turbulence measurements made by thereceiver 110 to account for the motion of the mobile platform (i.e. theantenna 138) and for ionospheric effects on the signal 108. The motionof the antenna 138 is caused by a combination of the velocity of themobile platform (in any combination of the x, y, and z dimensions) aswell as the rotation of the mobile platform about its roll, pitch, andyaw axes. Thus, the received signal may include alterations(particularly phase and intensity variations) caused by the motion ofthe antenna 138. Accordingly, the INS/FCS system 140 provides thereceiver 110 a signal that conveys the 6 degree of freedom (6 DOF)motion of the mobile platform to the receiver 110 via the input 127. Aphase shift estimator 128 and a frequency shift estimator 130 act on thevelocity data to determine the phase and frequency alterationsintroduced into the received signal because of the mobile platformmotion. More particularly, the steady-state linear velocity of theaircraft 122 and the associated Doppler effect is easily determined bythe frequency estimator 130. Because the steady state velocity isrelatively constant, any phase difference introduced by the steady statevelocity generally will contribute little to the measured turbulence inthis manner. To the extent that the mobile platform velocity causes aphase shift, though, the phase shift is determined from the velocity bythe phase shift estimator 128. Similarly, the phase shift estimator 128determines the phase shift caused by the acceleration of the mobileplatform. Again, the phase difference arises because the signal arrivingat one instant travels a slightly different distance than a signalarriving at the next instant, with the distance changing in accordancewith the acceleration. Thus, the phase of the signal appears to shift byan amount determined by the travel of the mobile platform between thearrival of the signals at the different times.

In contrast to the linear velocity of the aircraft, the rotationalvelocity is subject to more rapid changes. These angular accelerationsarise from several sources including control inputs, local turbulenceexperienced directly by the aircraft, and aerodynamic forces acting onthe aircraft. Thus, the phase and frequency shift estimators 128 and 130use knowledge of the antenna locations and orientation on the aircraftalong with the sensed rotational motion to determine the Doppler andphase shifts caused by the instantaneous linear velocity andacceleration arising from the rotation. The inverters 129 and 131 invertthe resulting signals and communicate the result to the processor 126.The processor 126 then adjusts the signals that convey the magnitudesand rates of the alterations generated by the phase, frequency, andintensity detectors 120, 122, and 124 to remove the alterations causedby the motion of the antenna 138. The adjustment of the signal can be byway of, for example, a filtering algorithm. The adjusted magnitude andrate signals are then converted by the processor 126 to a measurement ofthe turbulence along the signal 108 path through the atmosphere.Accordingly, the processor 126 of FIG. 2 generates a measurement of theturbulence that is corrected for the motion of the antenna 138.

In addition to the alterations induced in the signal by troposphericturbulence, the ionosphere also alters the signal via interactionsbetween the signal and the charged particles in the ionosphere. Becauseionospheric scintillation is strongly frequency dependent, theionospheric scintillation detector 132 can, by comparing the L1 and L2GPS signals 108 (recall that the GPS system uses one signal at the L1frequency of about 1575 MHz and another signal at the L2 frequency ofabout 1228 MHz) to detect the amount of scintillation introduced intothe signal 108 by the ionosphere. The inverter 134 inverts the outputfrom the ionospheric scintillation detector 132 and communicates theinverted signal to the processor 126. The processor 126 uses theinverted ionospheric scintillation signal to remove the effects of theionospheric scintillation from the turbulence estimate. Thus, theprocessor 126 generates a signal indicative of the troposphericturbulence encountered by the signal 108 that is filtered of the effectsof the antenna motion and of the ionosphere.

Ionospheric scintillation is relatively constant with respect toelevation angle (i.e. the apparent height of a satellite above thehorizon) whereas tropospheric scintillation varies strongly withelevation angle. This relationship between elevation angle andtropospheric scintillation is an inverse relationship. Accordingly,ionospheric scintillation predominates at high elevation angles andtropospheric (turbulence induced) scintillation predominates at lowelevation angles. Thus, in a preferred embodiment, the antennas 138 andreceivers 110 are adapted to accept low elevation angle (less than aboutthe 5 degree default mask angle of the GPS system) signals.

At the next stage of the receiver 110 (as illustrated in FIG. 2),additional information is associated with the turbulence measurement. Inparticular, the direction finder 136 receives heading and orientationinformation from the mobile platform INS/FCS system 140 via the input127. Additionally, the direction finder 136 receives information fromthe signal conditioner 118 regarding which antenna 138A or 138B receivedthe signal 108 and which satellite 106 generated the signal. Theseantennas 138A and 138B correspond to the two antennas 23 and 25 on theaircraft 22 of FIG. 1. Knowing the location of each antenna on theaircraft 22 and the orientation of the antenna relative to the aircraft,the direction finder 136 determines the direction to the satellite 106that transmitted the signal 108 received by the antenna 138A or 138B.The direction finder 136 of the current embodiment associates thedirection and the time that the signal 108 was received with theturbulence measurement which it receives from the processor 126.Accordingly, the output generated by the direction finder 136 is a timevarying vector defined by the amplitude of the turbulence measurement(from the processor 126) and the direction (in three dimensions) foundby the finder 136. This turbulence vector reflects the total amount oftropospheric turbulence along the signal 108 path at the time of thesignal's 108 receipt.

FIG. 1 also shows another preferred embodiment that includes theaircraft 22 which has three antennas 23, 25, and 27. Each of theantennas communicates with a receiver, such as the receiver 110 of FIG.2, for the measurement of turbulence. As shown, the aircraft 22 isflying toward the right and has the antenna 23 and 25 spaced apart fromeach other by a distance generally in the direction of the aircraft'svelocity. The antennas 23 and 25 are preferably on the sidewalls of theaircraft 22 and look abeam from the aircraft 22. The antenna 27 islocated at the nose of the aircraft 22 and faces forward along thedirection of travel. As the aircraft 22 moves, the paths 36 and 38between the antennas 23 and 25 and the satellite 12 also move while thereceiver 110 continues making turbulence measurements. As the paths 36and 38 move, the paths move into, through, and out of the various areasof turbulence 30 in the atmosphere. In contrast, because the antenna 27looks forward, the paths leading to the antenna 27 from most satelliteswill move very little as a result of the aircraft's motion (althoughthey will shorten as the aircraft moves toward the satellite).Accordingly, the side facing antennas 23 and 25 will receive signalsthat have more apparent turbulence induced variations than the signalsreceived by the forward facing antenna 27.

Over a period of time Δt, the aircraft 22 moves by a certain distancefrom the location where the leading antenna 25 received the signal alongpath 38 to a location where the trailing antenna 23 receives the signalalong the path 36 which is located where path 38 was located. For anantenna separation of about 10 meters at a typical aircraft cruise speedof 200 meters per second, Δt is approximately 50 milliseconds. Turbuleslarge enough to cause measurable changes in the GPS signal typicallyvary on a much slower time scale. Thus, aside from changes in theturbules themselves, the trailing antenna 23 will receive the signal atthe end of the period Δt with approximately the same alterations made toit by the turbules that (previously) the leading antenna 25 received atthe beginning of the period Δt. That is, the measurement of turbulencemade by antenna 25 along path 38 will be about the same as themeasurement of turbulence made by the antenna 23 along path 36.

In reality, various error sources will likely cause mismatches betweenthe measurements made by the two antennas 23 and 25. However, most ofthe error sources will either be truly random (e.g. thermal noise in thereceiver 110) or they will be common to both antennas (e.g. timingvariations aboard the GPS satellite). In the latter case, the errorswill be simultaneous but will occur at different locations. That is,simultaneous errors common to both antennas 23 and 25 will affect themeasurement made by antenna 23 along path 36 and will affect themeasurement made by antenna 25 along path 38. During both the previousand subsequent measurement cycles, the measurements along both paths 36and 38 will likely be unaffected.

To eliminate the random and common mode errors, the receiver 110correlates the two time-sequences of data resulting from themeasurements made by the two antennas 23 and 25. One of the twotime-sequences includes the samples of turbulence-related data (e.g.amplitude changes, phase shifts, or frequency shifts) from the leadingantenna 25. The other time-sequence of turbulence data is collected fromthe trailing antenna 23 and delayed with respect to samples in the firstsequence taken at the same location by Δt. Accordingly, the magnitude ofthe coefficient of correlation, r(Δt), for these two time-sequences ismaximized for parameter changes caused by turbules on the scale of thespacing between the two antennas 23 and 25. The correlation coefficientwith Δt≠0 also minimizes the effect of random errors in the two sets ofdata.

In a preferred embodiment, the receiver 110 of FIG. 2 continuouslydetermines the coefficient of correlation, r(Δt), and provides an outputsignal proportional to r²(Δt). This output can be used as an indicatorof turbulence along the line of sight to the satellite and is morerobust against error than an indicator based on a single antenna (e.g.antenna 23 alone). In other preferred embodiments, the inventionprovides more than two GPS antennas along the length of a largeaircraft. Because many aircraft already have redundant antennas, littleor no equipment need be added to these aircraft. In these embodiments,the receiver 110 computes a coefficient of correlation for themeasurement data sets obtained by all of the antennas. The time-sequencefor each antenna is delayed by an appropriate interval so that all datasets cover the same signal path.

In another alternate embodiment, the invention uses signals fromsatellites other than those satellites that are designed to provideprecise navigation signals. Examples include communication and weathersatellites. Candidate communication satellites include the satellites inthe Iridium, GlobalStar, ICO, and similar constellations. One of theadvantages of using these satellites is that they are more numerous thanpositioning satellites so they provide more frequent opportunities tomeasure turbulence along a particular line of sight or above aparticular region. For embodiments using communications satellites it ispreferred that the receiver correlate the signals from two or moreantennas so as to reject variations in the phase and frequency of thetransmitted signals that can be caused by timing errors in thesatellites' clocks.

With reference now to FIG. 3, another system 200 constructed inaccordance with the principles of the present invention is illustrated.FIG. 3 differs from FIG. 2 by generally showing how the system 200distributes and uses the turbulence vectors generated by the receivers210 whereas FIG. 2 generally illustrates how the receivers 110 generatethe turbulence vectors. Briefly, the transmitter 206 transmits signalsto the antennas 238. Systems 240 on the mobile platforms provide thereceivers 210 with information regarding the mobile platforms' velocity,heading, and orientation. From these signals, the receivers 210 generatethe turbulence vectors while, preferably, adjusting the as-receivedsignals for the velocity of the mobile platforms on which the receivers210 are situated. FIG. 2 also illustrates the receivers 210 providingseparate signals 254, 256, 258, and 260 carrying information pertainingto, respectively, the ionospheric scintillation, the troposphericturbulence, the correlation between different measures of thetropospheric turbulence, and the directions in which each of theturbulence measurements was made.

FIG. 3 also shows several additional aspects of the current embodimentincluding a network 262, a computer or processor 264, a meteorologicalprediction model 265, a set of air data sensors 266, a set ofmeteorological sensors 268, a set of inertial sensors 270, and apopulation of subscribers 272 that includes the Air Traffic ControlSystem 274. The processor 264 receives the numerous turbulence vectorsand related information over the network 262 which may include anairborne network such as the Connexion by Boeing^(SM) system. From theturbulence information, the processor 264 creates a three-dimensionalmodel of the turbulence measured by the numerous receivers 210.Preferably, the processor 264 executes a tomography algorithm on thecollection of turbulence vectors to yield the three-dimensional model.

Tomography is a set of processes for determining the two-dimensional orthree-dimensional distribution of a quantity from a set of measurementsof that quantity taken along paths through an object or volume. Typicalproducts of tomographic processes include cross sectional depictions ofthree dimensional objects. An example of tomography is ComputerizedAxial Tomography (CAT), the basis of medical CAT scans. During a CATscan, the quantity measured is x-ray absorptivity as a proxy for tissuedensity. The CAT scan measures total x-ray absorption along each of manypoint-to-point lines through the patient's body. The tomographicalgorithm uses the collection of these one-dimensional x-ray absorptionmeasurements to estimate the x-ray absorptivity at many points insidethe body. Then, the CAT scan machine displays those measurements in atwo-dimensional depiction or a three dimensional, electronic model ofthe structures that absorbed the X-ray.

Referring again to FIG. 1, each of the paths 36, 38, 40, 42, 44, and 46represents a single, one-dimensional measurement of the turbulence 30 inthe atmosphere. These measurements may be adjusted to remove the effectsof ionospheric scintillation and the movement of the transmitter orreceiver. Also, the transmitting satellites 12, 14, 16, 18, and 20 andmobile platforms 22, 24, and 26 shown move thereby causing the signalpaths to sweep through the atmosphere. The movement of the paths 36, 38,40, 42, 44, and 46 allows many measurements of the turbulence 30 for anypair of one transmitter and one receiver. It should also be noted thatthe paths (not shown) between the ground station 28 represent a specialcase in which the paths move but pivot around one fixed end at theground station 28. Since the turbulence 30 moves and evolves at a slowerrate than the rapidly moving satellites 12, 14, 16, 18, and 20 andmobile platforms 22, 24, and 26, the measurements will remain valid forsome time after they are taken. Further, since approximately 5,000aircraft are aloft during a typical peak hour of flight time in theUnited States alone, and since there are at least 4 GPS satellitesvisible from any location, the system of FIG. 1 allows multiples of20,000 measurements of the turbulence 30 over the United States duringthe hours of most interest for detecting turbulence 30. This roughestimate does not include many types of potential receivers (e.g.handheld receivers, marine vehicles, land vehicles, stations, and theirequivalents) and many types of potential transmitters (e.g. otherpositioning system satellites, communication satellites and theirequivalents) so the actual number of potential measurements issubstantially greater the 20,000. All of these receivers (i.e. samplingnodes) are in communication with the processor 264 via the network.Since the processor 264 communicates via the network 262 its location isnot critical and could even be onboard one of the mobile platforms orsampling nodes.

In operation, each sampling node continuously measures the troposphericturbulence 30 along the line of sight from the node 22, 24, 26, or 28 toone, or more, of the transmitting satellites 12, 14, 16, 18, and 20. Thesampling nodes 22, 24, 26, or 28 transmit their one-dimensionalturbulence measurements, including the locations, directions, and timesassociated with each measurement to the processor 264. To build themodel, the processor 264 examines the set of measurements and identifiespoints, or volumes, where turbulence 30 is present. FIG. 1 shows howthis process operates on a relatively small sample of measurements. Asillustrated, many of the paths 36, 38, 40, and 44 will miss any giventurbule 30 in the atmosphere. However, other paths 42 and 46 willintersect the turbule 30 resulting in corresponding measurments thatwill be marked by a high degree of scintillation. By examining each ofthe many pairs of paths 36, 38, 40, 42, 44, and 46 to determine whetherthey intersect (or nearly intersect) and whether both paths exhibit highturbulence, the processor 264 identifies volumes of turbulence 30 at theintersection, or “near” intersection, of the pair of paths (here paths42 and 46). A near intersection means that the paths do not necessarilyintersect, but rather, pass within a distance from each other on thescale of the turbules 30 of interest. Once a path intersection with highindications of turbulence on both of the paths is identified, additionalpaths that come near the first intersection can be examined to improvethe identification and measurement of the turbulence 30. Other pathsthat intersect either of the first pair of intersecting paths 42 and 46can be examined to confirm that the measured turbulence actually occursat the intersection within the turbulence 30 rather than somewhere elsealong one of the intersecting paths 42 and 46. In other words, the factthat path 44 (for example) intersects path 46 but does not indicateturbulence, can be used to confirm that it is the intersection of path46 with path 42 about which the turbulence 30 can be found. In apreferred embodiment, a program for creating the model is stored on acomputer readable medium. The medium can be ROM, RAM, a hard drive, aCD, a floppy disk, flash memory, EPROM, mass storage, a network overwhich the program is transmitted, or any of their equivalents.

The sample of paths 36, 38, 40, 42, 44, and 46 shown in FIG. 1 isrelatively small but represents a much larger number of paths that wouldpreferably be used. However, the mobility of the transmitting satellites12, 14, 16, 18 and 20 and sampling nodes 22, 24, 26, and 28 allows alarge number of measurements to be made near the intersection of the twopaths 42 and 46 because the paths 42 and 46 move while the multiplemeasurements are made. Further, because the paths 42 and 46 willcontinue to intersect the turbulent volume 30 for numerous measurementsalong each path 42 and 46, the processor can identify the location ofthe turbule 30 by comparing the paths 42 and 46 in the time periodduring which they neared each other (and the turbule 30 also). Thus,when the processor detects an intersection of paths each having highturbulence, the processor can confirm the existence of a turbule 30 andits location by looking backward (and forward) along the time series ofmeasurements associated with the intersecting paths 42 and 46. As aresult, the present invention allows for a rapid initial localization ofturbules 30 followed by more thorough and accurate confirming checks ofthe initial estimate. Further, because each time series of measurementsfor a given path 36, 38, 40, 42, 44, and 46 over some time period can betreated statistically, the model can include a statistical confidenceinterval associated with the location of each turbule 30. Also,processing efficiency can be achieved by only comparing the paths 36,38, 40, 42, and 46 that intersect over a given region and by notprocessing those path intersections that occur above the troposphere 32or within the surface boundary layer.

Once the processor 264 builds (or modifies) the model, the network 262can be used to distribute the model. Preferably, the network 262includes a publisher-subscriber architecture that enables entities onthe network 262 to subscribe to the model with the processor 264 servingas the publisher. In this manner, bandwidth requirements fordistributing the turbulence model can be limited without compromisingthe quantity or quality of information being made available to thesubscribers 272. Additionally, the model can be segmented according topre-selected geographic areas over which the turbulence 30 occurs sothat the subscribers 272 can subscribe to geographic subsets of theoverall information contained in the model. The presence of GPSequipment already onboard many of the subscribers (e.g. aircraft thatmight also be measurement nodes) makes the implementation of locationbased subscription services easily achievable over the network 262.Additionally, conventional air-to-ground bidirectional communicationsystems (e.g. radios) can be used to relay turbulence relatedinformation between the components of th system. Thus, warnings ofturbulence can be transmitted from the ground to aircraft in thevicinity of the turbulence other than the aircraft that measured theturbulence. If the aircraft that measured the turbulence might beaffected by the turbulence onboard systems can communicate theturbulence information to the aircrew, or autopilot, so that appropriateevasive action can be initiated.

One type of subscriber 272 of particular interest is the Air TrafficControl (ATC) system 274 of the United States and its counterparts inother nations. The turbulence model can be distributed to the ATC system274 where it can be further distributed to the Control Centers and AirTraffic Control Towers (ATCTs) for use in controlling air traffic.Another exemplary subscriber 272 is the National Weather Service whichcan make use of the model for predicting severe weather. In otherpreferred embodiments, the subscribers 272 can include display devicesthat allow tomographic turbulence information to be overlaid onnavigation displays.

In other preferred embodiments, the processor can augment the model withdata from other sources. For instance, the meteorological model 264 canprovide estimates of the turbulence in volumes of the atmosphere wherethe signal paths between the transmitters and receivers have not sweptfor some time. Also, each aircraft (or mobile platform) thatcommunicates with the system 200 will typically be outfitted with airdata sensors 266. Because the air data sensors provide contemporaneous,localized, turbulence measurements, the air data sensors 266 canconfirm, or augment, the information in the turbulence model. Anotherexemplary source of information is the inertial sensors 270 onboard themobile platforms. Again these sensors 270 directly and contemporaneouslymeasure turbulence that the system otherwise senses remotely. Likewise,the system 200 can augment the turbulence model with meteorologicalinstruments 268 (e.g. weather stations) in areas prone to infrequentsignal sweeps. Thus, the collection of sensors 266, 268, and 270 can beused to calibrate and adjust the model in addition to merely augmentingthe information distributed via the model.

With reference now to FIG. 4, a method 310 in accordance with theprinciples of the present invention is illustrated. Generally, themethod 310 includes receiving electromagnetic energy that has beenaltered by turbulence, detecting the alteration caused by theturbulence, and building a three-dimensional model of the turbulence.More particularly, FIG. 4 shows the energy being transmitted inoperation 312 and encountering turbulence in operation 314 as itradiates from the transmitter. Because of the turbulence, the phase orthe frequency of the energy shifts, or fading or enhancement occurs tothe energy, as shown by the alteration in operation 316. In operation318, the altered energy is received. Operations 324 and 326 show thereceiver being moved and reoriented respectively while its heading andlocation are determined in operation 324. The alterations to theelectromagnetic energy are shown as being detected in operation 322.Operation 328 shows ionospheric scintillation being filtered from thesignal. Likewise, operation 330 removes the effects of receiver motionfrom the turbulence measurement. In operation 332 the direction,location, and time at which the energy was received are associated withthe measurement of the turbulence. If the turbulence was measured atmore than one location or time, the measurements can be correlated as inoperation 334. Once enough measurements of the turbulence are gatheredto allow for a statistically meaningful model (as indicated by operation336), a three-dimensional model of the turbulence is created inoperation 338. Additionally, the model can be augmented with otherrelevant information such as meteorological data or meteorologicalpredictions in operation 340. Further, the turbulence model can bedistributed to end users as shown by operation 342.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated.

As various modifications could be made in the constructions and methodsdescribed and illustrated without departing from the scope of theinvention, it is intended that all matter contained in the descriptionor shown in the accompanying drawings shall be interpreted asillustrative rather than limiting. For example, instead of merelyavoiding turbulence, the detected turbulence can be used to advantage.In one exemplary embodiment, a mobile platform is positioned on theopposite side of the turbulence from a laser device to protect themobile platform from the laser. Similarly, the mobile platform canmaneuver so that a laser on board the mobile platform can hit a targetdespite the presence of the turbulence. Thus, the breadth and scope ofthe present invention should not be limited by any of the exemplaryembodiments, but should be defined in accordance with the claims andtheir equivalents.

1. A receiver of electromagnetic energy that travels along a path and isincident on an antenna, the antenna being responsive to incidentelectromagnetic energy to produce a received signal, the path beingsubject to turbulence, the receiver comprising: an input to accept thereceived signal; a circuit in communication with the input, the circuitto accept the received signal and to accept a signal that isrepresentative of a velocity of a mobile platform, at least one of thereceiver and a source of the electromagnetic energy being on the mobileplatform, the circuit to process the received signal using the velocitysignal to remove a shift associated with the received signal because ofrelative motion between the source and the receiver, the circuit therebydetermining an alteration of the electromagnetic energy caused by theturbulence; and an output in communication with the circuit to output asignal that is representative of the turbulence.
 2. The receiveraccording to claim 1, further comprising the circuit to accept a signalthat is representative of a heading of the mobile platform and todetermine a direction to the source of the electromagnetic energy usingthe heading signal.
 3. The receiver according to claim 1, wherein thevelocity includes at least one of a linear velocity or an angularvelocity.
 4. The receiver according to claim 1, wherein the turbulenceincludes an ionospheric turbulence and a tropospheric turbulence, thereceiver further comprising the circuit to determine an alteration ofthe electromagnetic energy caused by the ionospheric turbulence and toprocess the received signal using the alteration of the electromagneticenergy caused by the ionospheric turbulence, the output signal beingrepresentative of the tropospheric turbulence.
 5. The receiver accordingto claim 1, wherein the receiver further comprises a global positioningsystem receiver.
 6. The receiver according to claim 1, furthercomprising a coupler to couple the receiver to the mobile platform. 7.The receiver according to claim 1, the received signal beingrepresentative of the incident electromagnetic energy at a firstlocation and being a first received signal, the alteration being a firstalteration, the circuit further comprising being adapted to accept asecond received signal from a second antenna, the second antenna beingat a second location, the circuit to process the second received signalusing the velocity signal to remove a shift associated with the secondreceived signal because of relative motion between the source and thereceiver, the circuit thereby determining a second alteration of theelectromagnetic energy caused by the turbulence.
 8. The receiveraccording to claim 7, further comprising the circuit to correlate thefirst alteration of the electromagnetic energy and the second alterationof the electromagnetic energy.
 9. The receiver according to claim 8,further comprising the output signal being further representative of thecorrelation between the first alteration of the electromagnetic energyand the second alteration of the electromagnetic energy.
 10. Thereceiver according to claim 1, wherein the source is a satellite. 11.The receiver according to claim 1, further comprising the circuit todetermine at least one of a phase difference, an intensity difference,or a frequency difference in determining the alteration of theelectromagnetic energy.
 12. A mobile platform comprising: at least oneantenna responsive to incident electromagnetic energy to produce areceived signal, the electromagnetic energy to travel along a path thatis subject to turbulence; a sensor to determine the velocity of themobile platform; and a receiver in communication with the antenna andthe sensor, the receiver including: an input to accept the receivedsignal and to accept a signal from the sensor that is representative ofthe velocity; a circuit in communication with the input, the circuit toprocess the received signal using the velocity signal to remove a shiftassociated with the received signal because of relative motion between asource of the electromagnetic energy and the receiver, the circuitthereby determining an alteration of the electromagnetic energy causedby the turbulence; and an output in communication with the circuit tooutput a signal that is representative of the turbulence.
 13. The mobileplatform according to claim 12, further comprising a sensor to determinea heading of the mobile platform, the input to accept a signal from theheading sensor that is representative of the heading, the circuit todetermine a direction to a source of the electromagnetic energy usingthe heading signal.
 14. The mobile platform according to claim 12, thealteration being a first alteration, the antenna being a first antennaat a first location on the mobile platform, the received signal being afirst received signal, the mobile platform further comprising a secondantenna responsive to incident electromagnetic energy to produce asecond received signal, the second antenna being at a second location onthe mobile platform, the circuit to accept the second received signaland to process the second received signal using the velocity signal toremove a shift associated with the received signal because of relativemotion between the source and the receiver, the circuit therebydetermining a second alteration of the electromagnetic energy caused bythe turbulence.
 15. The mobile platform according to claim 14, furthercomprising the circuit to correlate the first alteration of theelectromagnetic energy and the second alteration of the electromagneticenergy.
 16. The mobile platform according to claim 15, furthercomprising the circuit to further correlate the first alteration of theelectromagnetic energy and the second alteration of the electromagneticenergy using a time period defined by a distance between the firstlocation and the second location and the velocity signal.
 17. The mobileplatform according to claim 16, further comprising the output signalbeing further representative of the correlation between the firstalteration of the electromagnetic energy received and the secondalteration of the electromagnetic energy.
 18. The mobile platformaccording to claim 12, wherein the source is a satellite.
 19. The mobileplatform according to claim 12, wherein the mobile platform is anaircraft.
 20. The mobile platform according to claim 12, furthercomprising a sidewall, the antenna having a field of view that forms anacute angle with the side wall.
 21. The mobile platform according toclaim 12, wherein the antenna is adapted to accept low elevationsignals.
 22. A system for detecting turbulence comprising: at least onemobile platform; a sensor to determine a velocity of the at least onemobile platform; at least one transmitter to transmit electromagneticenergy along a path that is subject to turbulence; and at least onereceiver to receive the electromagnetic energy, at least one of thetransmitter and the receiver being on the mobile platform, the receiverincluding an antenna responsive to incident electromagnetic energy toproduce a received signal, the receiver to accept a signal from thesensor that is representative of the velocity, the receiver to processthe received signal using the velocity signal to remove a shiftassociated with the received signal because of relative motion between asource of the electromagnetic energy and the receiver, the receiverthereby determining an alteration of the electromagnetic energy causedby the turbulence; the receiver to output a signal that isrepresentative of the turbulence.
 23. The system according to claim 22,wherein a first mobile platform of the mobile platforms being selectedfrom the group consisting of an aircraft, a satellite, a land vehicle, amarine vehicle.
 24. The system according to claim 22, further comprisinga processor to receive the output signal and to create a threedimensional model of the turbulence using the output signal.
 25. Thesystem according to claim 24, wherein the three dimensional model is atomographic model.
 26. The system according to claim 24, furthercomprising a network in communication with the processor, the processorto distribute the three dimensional model of the turbulence via thenetwork.
 27. The system according to claim 26, the network furthercomprising a subscribe-publisher architecture.
 28. The system accordingto claim 26 further comprising an air traffic control system incommunication with the network.
 29. The system according to claim 24,wherein the processor determines a statistical confidence intervalassociated with the three dimensional model.
 30. The system according toclaim 24, further comprising at least one meteorological sensorcommunicating with the processor, the processor to determine ameteorological prediction based on a condition sensed by the sensor andto augment the three dimensional model of the turbulence with themeteorological prediction.
 31. The system according to claim 24, furthercomprising at least one meteorological model accessible by theprocessor, the processor to determine a meteorological prediction basedon the model and to augment the three dimensional model of theturbulence with the meteorological prediction.
 32. The system accordingto claim 24, the at least one mobile platform further comprising anmobile platform including at least one of an air data sensor or aninertial sensor, the at least one sensor in communication with theprocessor and to generate fourth signal, the processor to use the fourthsignal to augment the three dimensional model of the turbulence.
 33. Thesystem according to claim 22, wherein the at least one mobile platformincludes at least one of a ground vehicle or a marine vehicle.
 34. Thesystem according to claim 22, wherein the receiver is on the mobileplatform.
 35. The system according to claim 22, wherein the outputsignal includes a location of the receiver, a direction associated withthe electromagnetic energy, and a time associated with the reception ofthe electromagnetic energy.
 36. The system according to claim 22,wherein the transmitter is near the horizon as viewed from the receiver.37. The system according to claim 22, wherein the at least one mobileplatform is in a pre-selected geographic region.
 38. The systemaccording to claim 37, wherein the geographic region includes at leastone of an approach path to an airport or a departure path from theairport.
 39. The system according to claim 22, further comprising theantenna being a first antenna, the received signal being a firstreceived signal, the receiver including a second antenna, the receiverand the first antenna and the second antenna to be on the mobileplatform, the path being between the transmitter and the first antennaand being a first path, a second path being between the second antennaand the transmitter, the first antenna and the second antenna incommunication with the at least one receiver, the second antenna beingresponsive to incident electromagnetic energy to produce a secondreceived signal, the receiver to process the second received signalusing the velocity signal to remove a shift associated with the secondreceived signal because of relative velocity between the source of theelectromagnetic energy and the receiver, the receiver therebydetermining a second alteration of the electromagnetic energy caused bythe turbulence
 40. The system according to claim 39, further comprisingthe circuit to correlate the first alteration of the electromagneticenergy and the second alteration of the electromagnetic energy.
 41. Thesystem according to claim 40, further comprising the first antenna andthe second antenna being spaced apart by a distance, the circuit tofurther correlate the first alteration of the electromagnetic energy andthe second alteration of the electromagnetic energy using a time perioddefined by the distance and the velocity of the mobile platform.
 42. Amethod of predicting turbulence comprising: receiving electromagneticenergy that has traveled along a path subject to the turbulence, theturbulence altering the electromagnetic energy; and determining thealteration caused by the turbulence by filtering the electromagneticenergy as it was received with a velocity of one of a transmitter of theelectromagnetic energy or a receiver that received the electromagneticenergy.
 43. The method according to claim 42, further comprisingdetermining a direction between the receiver and the transmitter. 44.The method according to claim 42, wherein the velocity includes at leastone of a linear velocity or an angular velocity.
 45. The methodaccording to claim 42, the turbulence including an ionosphericturbulence and a tropospheric turbulence, the method further comprisingdetermining a portion of the alteration of the electromagnetic energycaused by the tropospheric turbulence.
 46. The method according to claim42, further comprising coupling at least one of the receiver and thetransmitter to a mobile platform.
 47. The method according to claim 42,wherein the receiving the electromagnetic energy being at a firstlocation, the alteration of the electromagnetic energy being a firstalteration of the electromagnetic energy, the method further comprisingreceiving the electromagnetic energy at a second location anddetermining a second alteration of the electromagnetic energy caused bythe turbulence by filtering the electromagnetic energy as it wasreceived at the second location with the velocity.
 48. The methodaccording to claim 47, further comprising correlating the firstalteration of the electromagnetic energy and the second alteration ofthe electromagnetic energy.
 49. The method according to claim 48,further comprising the correlating including accounting for a timedifference between the receiving of the electromagnetic energy at thefirst location and the receiving of the electromagnetic energy at thesecond location, the time difference being defined by the velocity and adistance between the first location and the second location.
 50. Themethod according to claim 42, the receiving the electromagnetic energyfurther comprising being from a satellite.
 51. The method according toclaim 42, the determining the alteration of the electromagnetic energyfurther comprising detecting at least one of a phase difference, anintensity difference, or a frequency difference.
 52. The methodaccording to claim 42, further comprising creating a three dimensionalmodel of the turbulence.
 53. The method according to claim 52, whereinthe three dimensional model is a tomographic model.
 54. The methodaccording to claim 52, further comprising distributing the threedimensional model via a network.
 55. The method according to claim 52further comprising at least one of subscribing to or publishing thethree dimensional model.
 56. The method according to claim 52, furthercomprising determining a statistical confidence interval associated withthe three dimensional model.
 57. The method according to claim 52,further comprising augmenting the three dimensional model with ameteorological prediction.
 58. The method according to claim 52, furthercomprising collecting turbulence data with a sensor in the turbulenceand augmenting the three dimensional model with the data.
 59. The methodaccording to claim 42, wherein the turbulence is associated with anairport.
 60. The method according to claim 42, further comprisingaccepting the receiving of the electromagnetic energy at a lowelevation.
 61. The method according to claim 42, further comprisingavoiding the turbulence that caused the alteration to theelectromagnetic energy.
 62. The method according to claim 42, furthercomprising issuing a warning based on the alteration that caused thealteration to the electromagnetic energy.
 63. A computer readable mediumhaving executable instructions stored thereon for causing the computerto: accept a plurality of one-dimensional measurements of turbulence,each of the plurality of measurements being taken along a path, at leastone measurement of the plurality of measurements being adjusted for avelocity associated with a system that made the at least onemeasurement; determine an approximate intersection of at least two ofthe paths; determine the amount of turbulence at the approximateintersection; and indicate the amount of turbulence at the approximateintersection.
 64. The medium according to claim 63, wherein a locationis associated with each of the measurements.
 65. The medium according toclaim 63, wherein a heading associated with the system is associatedwith each of the measurements.
 66. The medium according to claim 63,wherein the plurality of measurements have been adjusted to remove anaffect of ionospheric scintillation.
 67. The medium according to claim63, wherein the plurality of measurements are derived from a pluralityof GPS signals.
 68. The medium according to claim 63, wherein theplurality of one-dimensional measurements is a first plurality, themedium further having executable instructions stored thereon for causingthe computer to accept a second plurality of one-dimensionalmeasurements of the turbulence.
 69. The medium according to claim 68,further having executable instructions stored thereon for causing thecomputer to correlate the first plurality of measurements and the secondplurality of measurements.
 70. The medium according to claim 68, whereinthe first plurality of measurements and the second plurality ofmeasurements are separated by a time period associated with the velocityof the system.
 71. The medium according to claim 63 wherein theplurality of measurements are derived from at least one of a phasedifference, an intensity difference, or a frequency difference.
 72. Themedium according to claim 63 wherein an elevation angle is associatedwith each of the measurements, at least one elevation angle being low.73. The medium according to claim 63, wherein the model is a tomographicmodel.
 74. The medium according to claim 63, further having executableinstructions stored thereon for causing the computer to limit the modelto a pre-selected geographic limit.
 75. The medium according to claim63, further having executable instructions stored thereon for causingthe computer to accept turbulence information from a source external tothe model and to augment the model with the information.
 76. The mediumaccording to claim 75, wherein the source is at least one of ameteorological sensor, a meteorological prediction, an air data sensor,or an inertial sensor.
 77. The medium according to claim 63, furtherhaving executable instructions stored thereon for causing the computerto create an overlay for displaying the indication of the amount of theturbulence.
 78. The medium according to claim 63, further havingexecutable instructions stored thereon for causing the computer todetermine a statistical confidence interval associated with the amountof the turbulence.
 79. A computer readable medium having executableinstructions stored thereon for causing the computer to: accept areceived signal wherein the received signal is produced by an antennathat is responsive to incident electromagnetic energy, theelectromagnetic energy to travel along a path that is subject toturbulence; accept a signal that is representative of a velocity of amobile platform, at least one of the antenna and a source of theelectromagnetic energy being on the mobile platform; process thereceived signal using the velocity signal to remove a shift associatedwith the received signal because of relative motion between the sourceand the antenna thereby determining an alteration of the electromagneticenergy caused by the turbulence; and output a signal that isrepresentative of the turbulence.